The following U.S. Patents, which include patents of the present applicant, are believed to represent the current state of the art
U.S. Pat. Nos. 582,911, 1,266,637, 1,983,0612, 2,504,638, 2,550,313, 3,324,878; 3,756,269, 4,064,907, 4,177,829, 4,483,367, 4,541,464, 5,072,751; 6,026,841, 6,227,233 and 6,308,729